The present invention relates to a magnetic recording medium in which medium noise and thermal fluctuation are suppressed, and relates to a method for manufacturing the same.
Requirement of densification for a magnetic storage apparatus has been further increased in light of an increase in capacity, device miniaturization, reduction in the number of parts, and the like. In order to realize the densification in the magnetic storage apparatus, noise reduction in a magnetic recording medium is important in addition to enhancement of read sensitivity of a magnetic head performing recording/reproduction and improvement in a signal processing method for read back signals.
Presently, in a magnetic recording medium used in an in-plane recording system and mainly commercialized, a magnetic layer constituted of fine crystal grains mainly composed of Co has been used. In the in-plane recording system, recording is performed by magnetization in a substrate in-plane direction. In order to set a {11.0} plane of each magnetic crystal grain parallel to a substrate plane, Cr alloy crystal grains with a 001 direction preferentially oriented perpendicular to the substrate plane are used for an underlayer, and the magnetic crystal grains are controlled to be almost epitaxially grown on the Cr alloy crystal grains. Alternatively, in some cases, on the underlayer of a Cr alloy with the 211 direction preferentially oriented perpendicular to the substrate plane, each Co alloy crystal grain is controlled such that a {10.0} plane thereof is set parallel to the substrate plane. In either case, each magnetic crystal grain is controlled such that a C-axis thereof is oriented substantially parallel to the substrate plane. This is because the direction of the C-axis of the Co alloy crystal grain is an easy magnetization axis, and the magnetization is set to be directed to the substrate in-plane direction.
In order to realize the 001 orientation and the 211 orientation of the Cr alloy, a seed layer is formed between the substrate and the Cr alloy layer. For example, in a case of an Al—Mg alloy substrate, the preferred orientation in the 001 direction is realized by forming a film of the Cr alloy by sputtering at comparatively high temperature on a Ni—P film plated on the substrate. A method of forming a film of a Co—Cr—Zr amorphous alloy on a glass substrate and forming a film of the Cr alloy is also put into practice. In addition, there is a method of setting the Cr alloy to be 211 oriented using a film of a Ni—Al alloy formed at low temperature as the seed layer. As described above, since the magnetic crystal grains are almost epitaxially grown on the Cr alloy grown by controlling the crystal orientation, the shape and the grain diameter of the magnetic crystal grains are strongly affected by the shape and the grain diameter of the Cr alloy crystal grains. Generally, the individual shapes of the Cr alloy crystal grains or the Co alloy crystal grains are irregular, but are isotropic on the average.
Incidentally, in order to reduce medium noise, it is effective to make the grain diameter fine and reduce exchange interaction between the adjacent crystal grains. This is because, when the exchange interaction is small, the width of a magnetization transition region of a boundary portion between adjacent recording bits, which is one of factors of the medium noise, heavily depends on the crystal grain diameter in the magnetic layer. In order to make the crystal grain diameter fine, a method of making the crystal grain diameter of the underlayer fine is effective. This is because, as previously described, the magnetic crystal grains are almost epitaxially grown on the Cr alloy crystal grains, and the crystal grain diameter in the magnetic layer is heavily affected by the crystal grain diameter in the underlayer. For making the grain diameter of the underlayer crystal grains fine, reported were means of performing surface oxidization of the Co—Cr—Zr seed layer (IEEE Tran. Magn. Vol. 35, No. 5, (1999) 2640), means of adding B into the Cr alloy (IEEE Tran. Magn. Vol. 33, No. 5, (1997) 2980), and the like. It is possible to make the crystal grain in the magnetic layer fine by making the crystal diameter of the Cr alloy grain fine using those techniques. On the other hand, in order to reduce the exchange interaction among the grains, reported are means of segregating Cr in the crystal grain boundary, means of forming a physical space between the crystal grains, and the like. For realizing the former one, a magnetic layer obtained by adding B or SiO2 into the Co—Cr—Pt alloy was examined (Japanese Unexamined Patent publication No. 7(1995)-311929) and then put into practice. The addition of B into the magnetic layer has an effect of making the size of the crystal grains of the magnetic layer itself fine. A combination of such crystal grains of the magnetic layer and the fine underlayer crystal grains makes it possible to effectively make the grain diameter of the magnetic grains fine and reduce the exchange interaction between the grains.
Such a method for reducing the medium noise, not limited to the in-plane recording medium, is the same in a perpendicular magnetic recording system, in which the direction of the recording magnetization is directed to a direction perpendicular to the substrate plane. For the magnetic recording medium for use in the perpendicular magnetic recording system, examination has been made of a magnetic recording medium using a magnetic layer similar to that for use in the in-plane recording medium, which is constituted of the Co alloy magnetic crystal grains, a magnetic recording medium using a magnetic layer constituted of crystal grains of an artificial lattice structure having Co and Pd, or Co and Pt alternately laminated, and the like. In such perpendicular magnetic recording media, making size of the crystal grains to be fine and reducing the exchange interaction are very important for reducing the medium noise.
In order to promote the densification in the magnetic storage apparatus, it is necessary to make the crystal grains fine. However, along with making the crystal grains fine, a problem of a so-called thermal fluctuation becomes noticeable, in which the recorded magnetization is attenuated with time by a thermal energy of about room temperature. As a technology for exceeding a limitation due to the thermal fluctuation involved in the fine crystal grains, a so-called anti-ferromagnetically coupled (AFC) medium is put into practice. In the AFC medium, the magnetic layer is composed of two layers or more with Ru interposed therebetween. Effective grain volume is increased by utilizing anti-ferromagnetically coupling between the upper and lower layers, and thermal stability is secured. Furthermore, a vertical recording system allowed to increase in thickness of the magnetic layer was proposed and a lot of examination thereof is made. In the case of the vertical recording, the higher the recording density is, the recording magnetization is more stable. Accordingly, the vertical recording is considered to be essentially suitable for high recording density.
In U.S. Pat. No. 5,989,674, disclosed is a technology for increasing shape anisotropy of the magnetic grains by providing fine grooves (texture) with a roughness period of about 150 nm on the substrate and forming acicular grains with an aspect ratio of about 2 to 6.
One among the dimensions of each crystal grain having a correlation with the medium noise is a crystal grain diameter in a track direction (a circumferential direction of the disc substrate) of a recorded bit pattern recorded by the magnetic head. This is because the width of the magnetization transition region of the boundary between the adjacent recorded bits depends on the grain diameter in the track direction, and is hardly affected by the grain diameter in a track width direction (a radial direction of the disc substrate). Therefore, if the shape of the grains is controlled so as to be elongated in the radial direction of the disc substrate, the small grain diameter and the large grain volume can be obtained simultaneously, and a thermally-stable medium can be realized while keeping the medium noise small.
The U.S. Pat. No. 5,989,674 described that the shape of the acicular magnetic crystal is elongated in the disc radial direction by providing substrate texture. However, with regard to the columnar magnetic crystal used in the existing magnetic recording medium, the examination by the inventors of the present invention has revealed that even if the magnetic layer is formed on the substrate simply subjected to texture treatment with various underlayers interposed between the substrate and the magnetic layer, the crystal grains with a shape as deformed in a specific direction with respect to the substrate plane are not formed. Furthermore, in the above-described US patent, there is no detailed description regarding the fine grooves for keeping the grain shape within a proper range and there is no description regarding a relation between the average grain diameter in the disc radial direction and that in the disc circumferential direction.